The invention relates to a driver circuit of lamps with low cold resistance comprising a power transistor of which the collector-emitter path is connected to the lamp in a series circuit which lies between a positive terminal and a ground terminal of a supply voltage source.
Lamps in which the emitted light is generated by passing a current through a resistance filament which heats the filament to white heat have in the unconnected state a low cold resistance which is for example only 10% of the resistance in the incandescent state. At the moment of switching on such a lamp simulates a short-circuit so that at the switching-on instant a relatively high current begins to flow. The same case however occurs when a short-circuit is present parallel to the lamp, this latter case however frequently leading to destruction of the source delivering the battery current or at least of components of said source.
In a known driver circuit of the type mentioned at the beginning steps are taken with the aid of which the high switching-on current can be supplied but nevertheless in the event of a short-circuit a protective mechanism still comes into operation. An essential part of this known driver circuit is an integrated circuit of type UAF 1780 made by the company Thomson Semiconductor. This module includes a very complicated circuit arrangement which moreover requires an extensive external wiring. The module or chip contains a DC voltage converter which requires as external components a coil and an electrolytic capacitor. The maximum output current which is supplied to the lamp to be switched on can be set with an external resistor. With a capacitor likewise to be externally connected a delay time is set. If in the event of a short-circuit the current set is exceeded then after expiry of the delay time the output is switched to the currentless condition. This means that in the known driver circuit detection of the case of a short-circuit takes place solely in dependence upon the time so that in the event of a short-circuit before expiry of the fixedly set delay time the high short-circuit current always flows through the driver transistor. Under unfavorable circumstances this can lead to destruction of the driver transistor.
The invention is based on the problem of providing a driver circuit of the type mentioned at the beginning which with low circuit expenditure makes a detection of a short-circuit not dependent on time possible. The driver circuit to be provided is to be adapted to production as fully integrated circuit apart from having a small number of external components.
In the driver circuit mentioned at the beginning this problem is solved by an RC member comprising a resistor connected on one side to the ground terminal and a capacitor lying in series therewith and connected on one side to the positive terminal of the supply voltage source, a comparator which compares the voltage at the resistor of the RC member with the voltage at the collector of the power transistor and furnishes at an output a blocking signal for the power transistor when the voltage at the resistor is more negative than the voltage at the collector, and a limiting member for limiting the voltage at the resistor of the RC member to a voltage value lying above the saturation value of the power transistor.
The function of the driver circuit according to the invention is based solely on a voltage comparison and this means that the short-circuit detection no longer depends on fixedly set time intervals but solely on the detection of the exceeding of predetermined voltage differences. The lower resistance the short-circuit has and the higher the operating voltage the sooner the short-circuit is detected and the blocking signal disconnecting the power transistor supplied. Since the comparator compares voltages which are derived in effect from the operating voltage present momentary voltage peaks of the operating voltage do not effect a disconnection of the power transistor as long as the voltages are not large enough to lead to destruction of the power transistor.